Electrodes for use in electrochemical cells, such as fuel cells or batteries, are well known in the art. Techniques employed for manufacturing these electrodes typically involve the use of solvents to suspend an electrochemically active materials to be coated onto a substrate. The mixture of solvent and active materials has the consistency of a paste or slurry and includes active materials and a binder, such as polyvinylidenedifluoride (PvdF). The paste, or slurry, electrode mixture may then be applied to an appropriate current collector after which the solvent is removed, forming electrode material. The electrode material may then be cut to appropriate sizes. Recently, the electrode forming techniques that require solvents have posed formidable problems in view of current environmental legislation. The solvents used in the aforementioned processes present safety hazards, requiring costly storage, handling and disposal procedures.
U.S. Pat. No. 3,573,991 to Lenfant et al. discloses an electrostatic projection technique of forming an electrode. The electrostatic projection involves, inter alia, imposing a charge on a support, imposing an opposite charge on the particles of powdered material which is to be applied to the support as a layer, and fluidizing the charged particles of powder above the oppositely charged support. The powder is attracted to, and coats on, the support. Thereafter, the support is subjected to curing processes.
U.S. Pat. No. 3,591,421 to Schultze et al. discloses a plurality of processes for forming electrodes, one of which includes distributing a hydrophobic polymer powder within the pores of a porous substrate. The substrate is disposed within a chamber and the polymer powder is whirled up, within the chamber, and thrown against one side of the substrate, while an intermittent vacuum is applied. In this fashion, the distribution of powder material, within the substrate, may be arranged as desired, dependent upon the application for the resulting electrode. The substrate is then subjected to compression and sintering treatment.
U.S. Pat. Nos. 4,287,232 and 4,313,972, both to Goller et al., disclose a dry method for making an electrochemical cell electrode. The methods involve, inter alia, depositing a layer of dry carbon/hydrophobic polymer powder on the surface of a substrate by dispersing the powder in a cloud chamber over the substrate. The powder is pulled onto the substrate by a vacuum disposed thereunder. The electrode is subsequently compacted and sintered.
U.S. Pat. No. 5,158,722 to Ilic et al. discloses a process and apparatus for forming electrodes without solvents that includes, inter alia, introducing a dry powder mixture of electrochemically active materials and a binder into a metal mesh, which serves as a current collector. The dry powder mixture is introduced into the metal mesh from opposing sides. To that end, two strips of the dry powder mixture are simultaneously rolled out from opposite sides of a gap, developed between a roller, while a lattice, or net-like, current collector enters therebetween. In this fashion, the current collector is coated on both sides with the dry powder mixture, filling the interstices of the metal mesh. A problem with the aforementioned electrode forming machine concerns the controlled deposition of the active materials.
What is needed, therefore, is a method and system for continuously impregnating active materials into a moving substrate while actively controlling the density distribution of the active materials disposed therein.